<computer hardware> A register found in some CPUs, whose contents can be added to the address operand to give the effective address. Incrementing the index register then allows the program to access the next location in memory and so on, making it very useful for working with arrays or blocks of memory.

Index registers first appeared around April 1949 in the Manchester Mark I. The Mark I's index register's contents were simply added to the entire instruction, thus potentially changing the opcode (see The story of Mel)!